Owing to the special physical properties of diamonds, especially to their extreme hardness, a large percentage of the natural rough diamonds mined are used in technical applications, such as, for example, in tools for machining high-hardness materials by grinding, drilling or cutting. The high demand for diamonds for technical purposes has led to the development of a variety of methods for the fabrication of industrial diamonds and the application of thin layers of synthetic diamond to substrates. Synthetically manufactured diamond coatings are used primarily to increase the hardness of tools for applications in light engineering and microtechnology.
Thin layers of diamond can be created by means of the CVD (chemical vapor deposition) method or by plasma-supported processes. This process technology makes it possible to produce synthetic diamond coatings at low pressures of roughly 0.01 to 100 torr and at moderate substrate temperatures of approximately 800.degree. to 1000.degree. C.
European Patent Application 0 413 394 describes a method for the production of polycrystalline diamond coatings wherein diamond crystallite is precipitated by CVD onto a substrate heated to a temperature in the range of 450.degree. to 1200.degree. C., under pressure in the range of 10.sup.-5 to 1 bar, from a gas phase containing hydrogen and, in a ratio of up to 30%, a carbon-containing gas. To accomplish this, the substrate is brought into contact with a gas phase whose energy content varies with time.
At least at the beginning of the deposition process, the energy content of the gas phase must be suitable for the nucleation of diamond crystallites in order for diamond crystallites to form on the surface of the substrate. For additional diamond crystallite to be formed in situ on the known substrate surface, the substrate must be brought into contact with a gas phase whose energy content is elevated in comparison to the initial state.
"Combustion Synthesis of Diamond," W.A. Yarbrough et al., Surface and Coatings Technology 39/40 (1989), pp. 241-252, proposes the use of an unmodified oxygen-acetylene soldering and welding torch to fabricate diamond coatings. Because of the high temperature of the gas, the substrate or substrate holder is water-cooled.
The torch is operated at a pressure of 25 to 50 torr and with an oxygen-to-acetylene ratio of roughly 1:16.
European Patent Application 0 379 994 describes a method for the deposition of polycrystalline diamond coatings on a coolable substrate wherein a C.sub.2 H.sub.2 --O.sub.2 undercoat is used. The design of the torch and the rate of combustion are not specified in this document.
European Patent Application 0 324 538 describes a method for the deposition of polycrystalline diamond by incomplete combustion in a C.sub.2 H.sub.2 --O.sub.2 flame. Here again, neither the design of the torch nor the rate of combustion is specified.
These documents uniformly describe the use of open acetylene-oxygen flames produced by conventional autogenous torches to deposit synthetic diamond coatings.
U.S. Pat. No. 2,714,563 describes a detonation gun and its mode of operation. A single fuel charge or a rapid succession of fuel charges whose composition permits detonation are fed into a gun and then ignited, in order to establish a single detonation or a series of detonations occurring at short time intervals. Particles such as powder are introduced into this gun in such a manner that they are accelerated by the detonation and its associated phenomena and are projected from the open end of the gun onto a surface. A device by which the above-described method can be implemented will be referred to hereinbelow as a detonation gun.
The object of the invention is to provide a method by which synthetic diamond coatings can be produced on substrates rapidly and at low cost, especially with minimal and low-cost equipment. The method according to the invention is also intended to reduce the production costs of synthetic diamond coatings.
In a first method, according to the present invention the substrate to be coated is placed directly in the cooled, (preferably water-cooled), detonation chamber of a high-velocity burner system. The substrate can be a metal or metalloid substrate, such as a silicon wafer, for example.
However, the substrate material can also be, for example, metal oxides, mineral substances or a ceramic. The substrate is either fixedly secured or movably disposed, depending on the application. The detonation chamber is filled with suitable oxidation preventing gases hereinafter referred to as inert gases, i.e., an atmosphere is initially created which prevents oxidation of the substrate. The substrate is heated to a preselected process temperature and maintained as constantly as possible at this temperature. This can be accomplished, for example, by electrical resistance heating or induction heating. The atmosphere in the detonation chamber prevents the surface to be coated from oxidizing during the heating of the substrate.
The detonation chamber possesses one or more inlet valves for the fuel gases and outlet valves for the exhaust gases, which open into discharge conduits. The inlet and outlet valves are actuated via a camshaft in coordination with the work cycle of the process, especially the detonation frequency. Connected upstream from this detonation chamber is a gas mixing chamber, which also has one or more camshafts to control the inlet and outlet valves for the fuel gases, which are opened or closed synchronously with the process and with the inlet and outlet valves of the detonation chamber. The actual coating of the prepared, heated substrate surface takes place as follows:
One or more hydrocarbon gases such as, for example, acetylene or propane gas, and oxygen, either premixed or separately premixed, are injected into the gas mixing chamber through the open inlet valves. In the case of acetylene and oxygen, the ratio of the components of the mixture can range from 1:1 to roughly 2:1, while with the use of propane gas-oxygen mixtures, the mixture ratio varies from 1:2 to 1:4.35. At this stage the inlet valves between the gas mixing chamber and the detonation chamber are closed, so that a chamber pressure of, for example, 1.5 bar prevails in the gas mixing chamber when the system is being operated on acetylene and oxygen, and 4.5 bar, for example, with the use of a propane gas-oxygen mixture. The inlet valves of the gas mixing chamber are initially closed to permit charging with the fuel gases. In the next step, the outlet valves from the detonation chamber to the discharge conduits are opened. This allows argon and other inert gases to escape through the discharge conduits.
The inlet valves between the gas mixing chamber and the detonation chamber are then opened, so that the carbon-rich detonation mixture can flow into the detonation chamber with its reduced pressure, and thereby ignite at the preheated substrate, burning explosively. Carbon in the form of graphite is thus deposited on the substrate surface and is transformed into diamond crystals due to the high substrate temperature and the detonation pressure. The combustion gases, heated to a high temperature, flow out through the discharge conduits, the outlet valves being open. After this operation, the inlet valves between the gas mixing chamber and the detonation chamber are closed again and the process is repeated as described, in accordance with the preselected detonation frequency and process duration.
With a second method according to the invention, synthetic diamond coatings can also be produced on substrates outside the detonation-combustion chamber by means of a high-velocity hydrocarbon-oxygen flame from a high-velocity burner system or an intermittent detonation flame generated by a detonation gun. The substrate to be coated is rotatably mounted, for example in a barrel open at both ends. This barrel is designed as a protectively jacketed barrel, for this application as a double-jacket or triple-jacket barrel of radial chamber construction. A first jacket or first chamber is used for the inert gas, and the second jacket or second chamber for the coolant. The rotatably or axially movable substrate situated in the center of the protective barrel is first heated to the desired process temperature, of approximately 450.degree. to 1200.degree. C., for example inductively or by electrical resistance heating, while inert gas flows out of the inner jacket of the protective barrel through axial bores. This inert gas prevents the oxidation of the substrate surface during the heating process. Next, either a high-velocity, surplus-gas flame is produced by means of a high-velocity burner system operated with a hydrocarbon-oxygen mixture, or an intermittent flame is generated by means of a detonation chamber. The substrate is so moved with a defined radial rotation and a proportionate axial course that the high-velocity flame or the intermittent detonation flame sweeps over the entire substrate surface to be coated while maintaining an adjustable distance therefrom. Between the flame outflow bore and the protectively jacketed barrel, the flame is outwardly shielded and conveyed into a water-cooled jacketed barrel.
Free carbon in the form of graphite precipitates out of the carbon-containing flame onto the preheated substrate surface and is transformed into diamond crystals as a result of the high pressure of the flame on the substrate surface exposed thereto. The substrate is maintained at a preselected constant temperature throughout the coating process. This can be accomplished, for example, by means of special cooling nozzles operated with liquid nitrogen or liquid CO.sub.2. Carbon dioxide (CO.sub.2) can be fed in through the central feed conduit in the nozzle system of the high-velocity torch or the detonation gun as support for the process.
To produce diamond coatings on large substrate surfaces, it is advantageous to work in a chamber filled with inert gas, in which the substrate is preheated to the appropriate process temperature of approximately 450.degree. to 1200.degree. C., for example inductively or by electrical resistance heating. The inert gas chamber contains either a high-velocity burner system or a detonation gun, each operated with a mixture of hydrocarbon gas and oxygen. Intermittent detonation flames can be generated with a detonation gun, in contrast to the continuously burning high-velocity flames created with a high-velocity torch. In both cases the preferred fuel gases are acetylene, propane gas and oxygen.
The mixture ratios of the hydrocarbon gases used and the oxygen are so adjusted as to produce a reducing surplus-gas flame, for example a mixture ratio of 1:1 to 1.8 when acetylene and oxygen are used as the fuel gas. The carbon-containing intermittent detonation flame or the continuously burning high-velocity flame is again passed over the substrate surface at a defined speed and at an adjustable distance. In the exposure area of the carbon-containing flame, carbon is deposited on the substrate, which is maintained at the process temperature as much as possible, and this carbon is transformed into diamond crystals under the high pressure of the flame. This process can be supported by the addition of very finely powdered graphite, which is introduced into the flame in combination with a suitable carrier gas.
In the method according to the invention, a high-velocity torch or a detonation gun is used instead of the conventional autogenous torches used in the methods of prior art. The flame pressure or detonation pressure on the heated substrate surface is 20 to 100 times higher than in previous technologies. This brings about much more rapid diamond crystallization than in the methods of prior art. As a result, thicker synthetic diamond coatings can be created with the same process duration, or, with the technology of the invention, the process duration can be reduced by 1000 to 2000% from that of the methods of prior techniques.
The method according to the invention can advantageously be implemented with the combustible hydrocarbon gases and noncombustible inert gases listed below: